Definitions
The definition of inertia.
What is the tendency of an object to resist changes in its velocity?
Newton's first law
What is 'an object in motion (or at rest) will tend to stay in motion (or at rest) until acted upon by an outside force?'
F=ma
F=1000 kg * 0.01 m/sec2 = 10 Newtons north
An astronaut in space throws a ball with an initial velocity of 2 m/sec east. What will the ball's velocity be in a month? What will the acceleration be?
2 m/sec east
zero
A Super Bowl player throws a football with a very strong force all the way down the football field. If the field were ice and there was no friction, what might happen to him? Why?
He would move in the opposite direction from the ball due to Newton's third law which states every action has an equal and opposite reaction.
The definition of friction.
What is a force that opposes motion, resulting from the contact of two surfaces?
DAILY DOUBLE
Newton's first law is often referred to as..
What is 'law of inertia'?
Someone wants to push a big box across floor.
Force of static friction is 10N west
Force of kinetic friction is 5N west
What force must he use to get it started?
Static friction is the friction opposing the initiation of motion so this is force needed to overcome to get the box moving. He would need to use a force of 10N east
A car travels 25 m/hr south and hits a tree. What direction does the driver go (forwards, backwards, no change, sideways)?
Suppose instead of hitting a tree, it hits another car head on at 30 m/hr north. What is relative velocity?
forwards
55 m/hr towards each other
A football player pushes a large training bag for practice. The bag weighs 50kg. Ignoring friction, if he accelerates at 3 m/sec2 south, what force is he exerting?
F=ma
F=50kg*3 m/sec2
F=150 N south
The definition of static friction.
What is friction that opposes the initiation of motion?
Newton's second law
What is 'when an object is acted on by one or more outside forces, the total force is equal to the mass of the object times the resulting acceleration.'
A car, mass 1000 kg, driving north runs out of gas and coasts to a stop at 10 m/sec2. What is the kinetic frictional force between the car and the road?
Ft=Fm-Ff
Since it is coasting, Fm = 0
Ft=0-Ff
Ft=ma= 1000kg*10m/sec2 = 10,000N
Ff = 10,000 N south
Someone pushes a car that is stopped. He has to push a lot more at first but once it starts he doesn't have to push as much. Why?
Because static friction is always bigger than kinetic friction.
A football player pushes a large training bag for practice. The bag weighs 50kg. The static friction is 17N north. The kinetic friction is 12N north.
a) What force must he use to get it to move?
b) Once it is moving, what force must he use to keep it moving at a constant velocity?
a) 17N south
b) Ft=Fm-Ff
Ft = ma
a is zero (constant velocity) so Ft = 0
0 = Fm - 12 N
Fm is 12 N south
What is kinetic friction?
What is friction that opposes motion once the motion has already been started?
Formula for force and its standard unit
What is F=ma and Newton
(where f is force, m is mass, a is acceleration)
Someone wants to push a big box,100 kg, across floor.
Force of static friction is 10N west
Force of kinetic friction is 5N west
Once the box is moving, what force must he use to keep the box moving at constant velocity east?
Ft = Fm - Ff
Ff = 5N since the box is moving (kinetic)
a = 0 since velocity is constant
Ft = ma = 100kg*0m/sec2 = 0N
0N = Fm - 5N
Fm = 5N east
A missionary pilot is delivering food and Gospel tracks to victims of a war. The plane is flying east towards the field of people. Should the pilot drop the supplies before the plane is over the people, when it is over the people or after the plane has passed the people?
The pilot must drop the supplies before the plane reaches the field. The supplies will have a velocity equal to that of the plane when they are dropped so they will continue to fly east as they fall. In order to hit the field, they should be dropped west of it.
DAILY DOUBLE
A large Kansas City Chiefs player who is running east plowed into a small 49ers player who was holding the ball and running west. Both players were were completely stopped and the 49ers player got a broken collar bone and the ball flew out of his hands. The KC player gets a bruise.
1) What direction does the ball go?
2) Explain Newton's 1st law as applies to this situation.
3) Explain Newton's 3rd law as applies to this situation.
1) the ball goes west
2) an object in motion (the ball) stays in motion until acted upon by an outside force. The ball was going west, it kept going west even when the 49er stopped.
3) For every action/force there is an equal and opposite reaction/force: force of the large KC player stopping and getting a bruise was equal to the opposite force of the small 49er stopping and getting a broken collar bone.
A Newton and its units.
What is the standard unit of force and is defined as kg.m/sec2?
Newton's third law
What is 'for every action, there is an equal and opposite reaction.'
Someone wants to push a big box,100kg, across floor.
Force of static friction is 10N west
Force of kinetic friction is 5N west
Once the box is moving, what force must he use to keep the box moving at 2 m/sec2 east?
Ft = Fm - Ff
Ff = 5N since the box is moving (kinetic)
Ft = ma = 100kg*2m/sec2 = 200N
200N = Fm - 5N
Fm = 205N east
A policeman fires a gun. Explain how you could see evidence of Newton's 3rd law.
The explosion of the bullet propels the bullet forward, and an equal force propels the gun back into your shoulder. Due to Newton's 3rd law, when you fire a gun and the bullet comes hurtling out of the front, an equal and opposite reaction means the gun recoils into your shoulder.
A super bowl fan eats 5 kg of chicken wings, 10 kg of nachos, and 5 kg of mozzarella sticks. He's a giant pig and eats all this at once with his hand moving at 2 m/sec2 towards his mouth, pushing the food across the table into his mouth. He exerts 50 N of force. What is the force of kinetic friction?
Ft = Fm - Ff
m = 5+10+5 = 20 kg
Ft = ma = 20kg*2 m/sec2 = 40 N towards his mouth
40N = 50N - Ff
Ff = 10 N away from his mouth